Revolution period around Sun: 164.8 Earth years, i.e. 60,182 Earth days

Temperature at pressure of 0.1 bar –218°C (55 K)

Moons: 14

The Neptunian system

Observations by the Voyager 2 probe and ground telescopes have enabled 14 moons to be identified.

The largest of these moons, Triton, spans 2,700 km and makes up 99.7% of the system’s total mass. It moves in a retrograde orbit around Neptune, meaning it is a Kuiper belt object captured by the planet. Its trajectory is also unstable due to tidal effects. It is gradually spiralling towards Neptune and will likely break up when it reaches the Roche limit1.

1 Edouard Roche was a 19th-century French astronomer who predicted that beyond a certain minimum distance from a planet, satellites would be torn apart by tidal forces. The rings of the giant planets in the solar system are beyond this limit, which explains why they can’t agglomerate into moons.

Credits: NASA.

Edouard Roche.Credits: Wikipedia.

Exploration

Voyager 2 is currently the only spacecraft to have flown by Neptune.

Launched on 20 August 1977, it flew past the planet on 25 August 1989.

Most of what we know about Neptune today has been deduced from the data obtained by this mission.